The technical community working in gas turbine engine technology have and are continually expending considerable effort to improve the cooling aspects of the engine's component parts, particularly in the turbine area. Obviously, improving the effectiveness of the cooling air results in either utilizing less air for cooling or operating the engine at higher temperature. Either situation attributes to an improvement in the performance of the engine.
It is axiomatic that notwithstanding the enormous results and development that has occurred over the years the state-of-the-art of film cooling and convection techniques are not optimum.
Some of the problems that adversely affect the cooling aspects particularly in vanes are (1) the pressure ratio across all of the film holes cannot be optimized and (2) in vanes that incorporate conventional inserts, the static pressure downstream of the insert is substantially constant. Essentially in item (1) above the holes that operate with less than optimum pressure drop fail to produce optimum film cooling and in item (2) above a constant stator pressure adversely affects internal convection.
One of the techniques that has been used with some degree of success is coating of the airfoil sections of the vanes with a well known thermal barrier coating. However, a coated vane conventionally requires drilling the film coating hole after the coating process by a laser which typically results in a cylindrical hole compromising the film cooling effectiveness, thus consequently reducing the effectiveness of the coating. Moreover, flow control through the laser hole is more difficult, presenting additional problems to the engine designer.
We have found that we can obviate the problems noted above and improve the cooling effectiveness by providing in the vane a plurality of pockets that form metering slots on the airfoil surface together with judiciously located holes associated with each pocket for feeding cooling air to the metering slots which, in turn, effectively coalesce the air into a film of cooling air that flows across the external surface of the vane. The passageway from these located holes to the inclined slots places the cooling air in indirect parallel flow heat exchange relation with the gas path.
It is contemplated within the scope of this invention that the vane be fabricated from either a total casting process or a partial casting process where a structural inner shell is cast and a sheath formed from sheet metal encapsulates the shell.
A vane constructed in accordance with this invention affords amongst other advantages the following:
1) Film cooling effectiveness is optimized.
2) The film cooling system can adapt to thermal barrier coatings and the like without film cooling compromise.
3) Convection is optimized since flow can be metered locally to heat-transfer requirements and overall pressure ratio.
4) In the sheet metal design a repair procedure can be accommodated where distressed panels can be replaced without scrapping the total part.
5) A pressure side or suction side panel of the sheet metal designed vane may be optimized for flow and film coverage.
6) Improved cooling is achieved with hole and slot sizes that are large enough to minimize internal plugging.
7) In the sheet metal configuration flexibility of material choices for the external shell is significantly increased.
8) In the fully cast configuration the vane can be cast in halves which offer the most versatility in terms of achieving desired cooling flows and film blowing parameters.